Harley Quinn joins forces with a singer, an assassin and a police detective to help a young girl who had a hit placed on her after she stole a rare diamond from a crime lord. A twisted tale told by Harley Quinn herself, when Gotham's most nefariously narcissistic villain, Roman Sionis, and his zealous right-hand, Zsasz, put a target on a young girl named Cass, the city is turned upside down looking for her. Harley, Huntress, Black Canary and Renee Montoya's paths collide, and the unlikely foursome have no choice but to team up to take Roman down.